1. Field of the Invention
This invention relates to the field of sails for boats and more particularly relates to sails having leading edges free from turbulent wake of upwind structures and having edge deflection restricting means constraining the sails to assume optimal airfoil shapes without twist.
2. Description of Prior Art
For centuries attempts have been made to improve the sailing characteristics and efficiency of sailing vessels. Such attempts still continue, receiving additional impetus by the popularity of sailboats for sport and recreation. Because there is a limit to the amount of sail a hull of a given size can carry, a considerable amount of attention has understandably been directed to improving sail efficiency.
The physics of sailing teaches that the propulsive force of sails, when sailing close to the wind, is derived more from horizontal sail lift than from push. Inasmuch as a sail acts much like an airplane wing, it follows that, for optimum efficiency, it should either be in the shape of, or constrained to assume a shape similar to, an airplane wing. Stated otherwise, the sails should have, or should assume under load, the cross section of a cambered airfoil.
Aerodynamic theory teaches that the flow of air impinging on an airfoil should be at a constant angle of attack along the leading edge of the airfoil, for optimum generation of lift. Airplane wings are made without twist; a desirable characteristic hitherto not achieved in sails, except for those downwind sails operating under regimes of fully-stalled airflow and not generating lift. Conventional mainsails of good design, well set, exhibit minimum twist angles of as much as half the difference between zero incidence and that incident angle at which stall begins. Only a portion of such sails can be set to a desired angle of attack of the incident wind.
A school of thought has developed which holds that twist is desirable, believing that since wind velocities aloft are sometimes greater than those below, lower angles of attack aloft compensate well for this velocity gradient. It would be easy to expand this reasoning to the absurd by advocating that sails be set just at stall along the foot and at zero incidence at the head. A more sensible approach would be to eliminate twist and design for smaller camber aloft, delaying flow separation while still generating lift aloft. Only when sails have readily-controllable twist is it possible to design for precise camber patterns with reasonable assurance that such patterns will usually be attained in use, for large amounts of twist will overcome the effects of precise camber control in the lofted sail.
Experience and testing teaches that any structure, such as a mast of circular section or even slender rigging, closely upstream or windward of the luff span of a sail causes a turbulent flow of air to impinge upon the sail and thereby considerably reduces sail lift. The angle of attack is defined as that angle between the flow of incident air and a straightline chord drawn between the leading edge of an airfoil section and the trailing edge in the plane of the airflow along that airfoil section. In a sail having curvature along the span, or height, the chords would most conveniently be drawn parallel to each other and the general direction of sail airflow, to avoid complications caused by local variations in airflow direction resulting from curvature along the span, sail attachments and hardware, rigging and other adjacent objects, etc. The general variation of the angle of attack along the span of a sail -- usually towards smaller angles near the head -- is called twist. For optimum sail lift the impinging air flow should be laminar, rather than turbulent.
In attempts to improve sail efficiency, various patents have disclosed use of rigid airfoil sails, similar to airplane wings, mounted vertically or off from vertical on a hull (e.g. Barkla, U.S. Pat. No. 2,804,038 and Smith, U.S. Pat. No. 3,295,487).
Other patents employ a more conventional fabric sail which has a luff free from supporting structure. Simpson, U.S. Pat. No. 2,756,711 and Berge, U.S. Pat. No. 2,944,505, for example, employ a tripod mast. Laurent, U.S. Pat. No. 3,173,395 employs a conventional single mast having a pivotally mounted boom which rotates a triangular sail about the mast to keep the sail upwind of the mast. Ryder, U.S. Pat. No. 2,147,501 discloses a pair of inclined masts which rotate with the sail, the masts being supported by a short spar pivotally mounted on a stub mast. Ellis, U.S. Pat. No. 3,626,883, employs a thwartship track for sliding the tack to windward of his mast.
To prevent sail billowing Robin, U.S. Pat. No. 3,195,494 discloses a sail tautly stretched in a triangular frame which is pivotally supported by a derrick at its top corner and by a stub mast at its lower edge. At least one patent (Malrose, U.S. Pat. No. 3,112,725) employs a tautly stretched sail and multiple battens, also to prevent sail billowing.
These and similar sail systems have serious disadvantages, however. A rigid airfoil sail is impractical because the boat is either constrained to sail in only one direction, or else it must be symmetrical fore and aft and be turned around to change tack. Rigid airfoils cannot be adjusted to wind conditions and, because of their generally greater weight aloft, cause the craft on which they are mounted to be unstable.
The other structures generally provide no way to prevent unwanted sail billowing, or if they do, it is at the expense of having upwind structures adjacent to the sail luff. Many are structurally impractical and many employ inefficient fore and aft symmetry. None of these other structures cause the sail to assume an airfoil shape without twist.
Heretofore, to the applicant's knowledge, there has been no disclosure of a free hanging fabric sail, free from the turbulent wake of upwind structures, to which an optimum airfoil shape is imparted by a sail-shaping batten, the sail support being such that the sail assumes a substantially uniform shape from head to foot without twist.